Refrigeration systems, particularly the evaporator coils of refrigerators, air conditioning equipment or heat pump systems, are susceptible to frost accumulation during operation. Frost accumulation is undesirable in that: it decreases the airflow (at times, completely) by blocking the air pass; it decreases the cooling efficiency of the system; it reduces the capacity of the system; and it raises operating cost. The accumulation of ice or frost on evaporator coils in refrigerating equipment is a significant problem. The accumulation insulates the air to be cooled from the cold refrigerating fluid circulating through the evaporator unit, resulting in difficulties in maintaining the refrigerated space at the desired temperature.
One known method for preventing frost accumulation utilizes for a time clock which at some pre-selected time interval switches off the compressor and turns on a thermal heating device adjacent to the evaporator coils to melt the ice or frost. The disadvantage of this technique is that ice or frost does not always accumulate at a constant rate, depending on the ambient humidity and temperature of the air. Further, most clock-actuated defrost systems defrost more often than necessary, out of abundance of caution, in order to eliminate all buildup of ice or frost. This defrosting on a regular time cycle, whether needed or not and, independent of the quantity of accumulated ice or frost, is inefficient and wastes electrical power.
Another method is to simply switch off the compressor for a predetermined length of time generally sufficient to allow the ice or frost to melt. However, the switching off of the compressor permits the temperature in the refrigerated space to rise, which only makes the compressor and its motor work excessively to maintain the desired temperature.
One known de-icing control method used for vehicle HVAC systems involves measuring the temperature at a fixed location in the system, such as in the fin area of an evaporator. For example, a low temperature limit controller or freezestat may be in the fin area which closes when the temperature drops to a predetermined temperature. Alternatively, a negative temperature coefficient (NTC) thermistor may be used. In either case, when it is detected that the temperature at the location of the sensor has dropped to a predetermined level, the compressor is switched off, and the air temperature raises. A predetermined temperature is established to be the maximum air temperature reached before the compressor is switched back on.
One problem associated with such method is that the sensor detects only a single location. A cold spot (i.e., a location cold enough to form ice), however, may occur at different locations over the entire surface, depending on refrigerant charge level, airflow distribution of the unit, airflow level, compressor on/off cycle, and so forth. Thus, the location of a cold spot is unpredictable. If a cold spot develops outside the sensing region, the fixed sensor is not able to sense the coldest spot, and the compressor will not switch off early enough to prevent frost accumulation. To alleviate this problem, the switch temperature may be set to a higher temperature than is optimum temperature (e.g., zero degrees Celsius), but this prevents air temperature from achieving its theoretical lowest temperature.
Another problem is that design limitations may render it difficult to select the proper core and mounting location for the temperature sensor. Further, in some applications, the selected core and system layout do not allow the temperature probe to pierce the core or fins.
Another method of de-icing used in vehicle HVAC systems utilizes measurement of the refrigerant temperature, for example, using an NTC thermistor. While this provides adequate operational results, sensor replacement requires complete removal of the refrigerant. Further, technique requires special precautions when mounting the sensor to prevent the possibility of leakage.
Still another method of controlling a vehicle HVAC involves measuring air side temperature, for example with an NTC thermistor. Such approach, however, does not provide an accurate representation of cold spots that may occur due to the dynamic load and condition of the refrigerant circuit. This problem is particularly acute in larger HVAC units, such as used in trucks.